Harmonic Structures: switzerland

In Switzerland, there's an old path, dating from roman times, which crosses the Viamala gorge. In 2005, after a previous wooden footbridge was destroyed by a landslide in 1999, a new bridge was built. It was placed in a new location, with less posibility of falling rocks.

The bridge spans 56 meters, about 70 meter above the valley, and quite a peculiar feature: it connects places with a height difference of 22 meter. It's not only a bridge, but a staircase.

Photo from lixelle

Photo from Marco Zanoli

It's a cable stayed bridge. The wood plattform (the bridge deck) hangs from steel cables which, as you already know, deform accordingly. Its funicular (the shape the cable adopts when it deforms), together with the steep stair, give rise to a powerful structure.

General forces: the weight of the bridge is supported by the upper cables in tension.

The two main steel cables, of just 36 mm of diameter, carry the load by means of tension to the foundations located in both sides of the river. Hanging from them, smaller cables (10 mm of diameter) hold the glue-laminated larch beams. Well, not really. That wouldn't be a good idea: instead, the secondary cables (those hanging from the main cables) are attached to transversal steel beams (HEA 120), and the wood beams are layed above them. This is in order to prevent tension perpendicular to the grain to appear and... believe me, you'd rather not do that. Have a look at the technical drawing here, to see how it is built.

The bridge deck is inclined, and it pushes on its lower support (that's ok, business as usual), but it pulls from the upper support. That's a different story: the bridge's own weight is trying to drag the foundation into the river. How can you avoid that?

The forces you have to stand if you don't want everything to fall down cliff!

Moreover: the cables pull inward (that is, deep into the gorge as well!) thus, something has to be done to prevent the fall: an outward force is more than required. The foundation is responsible for that and, apart from its design (have a look at it here), it must be really heavy. Its own weight is its only tool. Actually (have a look below, at the details section), the foundation of this apparently light bridge weighs seven times more than the bridge itself. It's not what you see, but also what you don't see.

Details